chapter 8 chapter 8 thermochemistry: chemical energy
TRANSCRIPT
Chapter 8 Chapter 8
Thermochemistry: Chemical Energy
Energy and Its ConservationEnergy and Its Conservation
Units:
1 Cal = 1000 cal = 1 kcal
1 cal = 4.184 J (exactly)
Energy: The capacity to supply heat or do work.
Kinetic Energy (EK): The energy of motion.
Potential Energy (EP): Stored energy.
Thermal Energy: The kinetic energy of molecular motion and is measured by finding the temperature of an object
Heat: The amount of thermal energy transferred from one object to another as the result of a temperature difference between the two
8.2 Internal Energy and State 8.2 Internal Energy and State FunctionsFunctions
First Law of Thermodynamics: The total internal energy E of an isolated system is constant - The sum of all the kinetic and potential energies of particles making up a substance
E = Efinal - Einitial
Systems and SurroundingsSystems and Surroundings
System = the object in question Surrounding(s) = everything outside the
system When both system and surrounding at same temperature thermal equilibrium When not Heat transfer to surrounding = exothermic (you feel the heat) hot metal!! Heat transfer to system = endothermic (you feel cold) cold metal!!
Internal Energy and State Internal Energy and State FunctionsFunctions
State Function: A function or property whose value depends only on the present state, or condition, of the system, not on the path used to arrive at that state
8.3 Expansion Work8.3 Expansion Work
3CO2(g) + 4H2O(g)C3H8(g) + 5O2(g)
7 mol of gas6 mol of gas
w = F x d = - P∆V
Expansion Work: Work done as the result of a volume change in the system
8.4 Energy and Enthalpy8.4 Energy and Enthalpy
Constant Pressure:
E = q + w
w = work = -PVq = heat transferred
q = E + PV
qP = E + PV
Constant Volume (V = 0): qV = E
Energy and EnthalpyEnergy and Enthalpy
= Hproducts - Hreactants
Enthalpy changeor
Heat of reaction (at constant pressure)
qP = E + PV = H
H = Hfinal - HinitialEnthalpy is a state function whose value depends only on the
current state of the system, not on the path taken to arrive at
that state.
8.6The Thermodynamic 8.6The Thermodynamic Standard StateStandard State
3CO2(g) + 4H2O(g)C3H8(g) + 5O2(g)
3CO2(g) + 4H2O(l)C3H8(g) + 5O2(g) H = -2219 kJ
Thermodynamic Standard State: Most stable form of a substance at 1 atm pressure and at a specified temperature, usually 25 °C; 1 M concentration for all substances in solution.
Standard enthalpy of reaction is indicated by the symbol ΔHo
3CO2(g) + 4H2O(g)C3H8(g) + 5O2(g) H= -2043 kJ
H° = -2043 kJ
Enthalpies of Physical and Enthalpies of Physical and Chemical ChangeChemical Change
Enthalpies of Physical and Enthalpies of Physical and Chemical ChangeChemical Change
Enthalpy of Fusion (Hfusion): The amount of heat necessary to melt a substance without changing its temperature
Enthalpy of Vaporization (Hvap): The amount of heat required to vaporize a substance without changing its temperature
Enthalpy of Sublimation (Hsubl): The amount of heat required to convert a substance from a solid to a gas without going through a liquid phase
Enthalpies of Physical and Enthalpies of Physical and Chemical ChangeChemical Change
2Al(s) + Fe2O3(s)2Fe(s) + Al2O3(s) Ho = +852 kJ
2Fe(s) + Al2O3(s)2Al(s) + Fe2O3(s)Ho = -852 kJ
endothermic
exothermicThe reverse reaction
Applying Stoichiometry to Applying Stoichiometry to Heats of ReactionHeats of Reaction
A propellant for rockets is obtained by mixing the liquids hydrazine, N2H4, and dinitrogen tetroxide, N2O4. These compounds react to give gaseous nitrogen, N2 and water vapor, evolving 1049 kJ of heat at constant pressure when 1 mol N2O4 reacts.
a. Write the thermochemical equation for this reaction
b. Write the thermochemical equation for the reverse of the reaction
c. How much heat evolves when 10.0 g of hydrazine reacts according to the reaction described in (a) ?
ExampleExample An LP gas tank in a home barbeque contains 13.2 kg of
propane, C3H8. Calculate the heat (in kJ) associated with the complete combustion of all of the propane in the tank
C3H8(g) + CO2(g) 3 CO2(g) + 4 H2O(g) ΔHo = -2044kJ
8.6 Calorimetry8.6 Calorimetry The process of measuring
heat transfer in at constant pressure (ΔH).
chemical/physical process qrxn + qsol = 0
qrxn = -qsol
Rrxn = system
Ssoln = surrounding
What you’ll do in lab Heat given off by rxn Measured by
thermometter Figure out qrxn indirectly
Calorimetry and Heat Calorimetry and Heat CapacityCapacityMeasure the heat flow at constant volume (E).
Calorimetry and Heat Calorimetry and Heat CapacityCapacity
Specific Heat: The amount of heat required to raise the temperature of 1 g of a substance by 1 °C.
q = (specific heat) x (mass of substance) x T
Molar Heat Capacity (Cm): The amount of heat required to raise the temperature of 1 mol of a substance by 1 °C.
q = (Cm) x (moles of substance) x T
Heat Capacity (C): The amount of heat required to raise the temperature of an object or substance a given amount.
q = C x T
Calorimetry and Heat Calorimetry and Heat CapacityCapacity
ExampleExample How much heat must be added to a 8.21 g sample of
gold to increase its temperature by 6.2 oC? The specific heat of gold is 0.13 J/goC.
ExampleExample When 1.045 g of CaO is added to 50.0 mL of water at
25.0oC in a calorimeter, the temperature of the water increases to 32.2 oC. Assuming that the specific heat of the solution is 4.18 J/goC and that the calorimeter itself absorbes a negligible amount of heat, calculate ∆H in kilojoules for the reaction
CaO(s) + H2O(l) Ca(OH)2(aq)
8.98.9Hess’s LawHess’s Law
8.9 Hess’s Law8.9 Hess’s Law
Haber Process:
Multiple-Step Process - Given
N2H4(g)2H2(g) + N2(g)
Hess’s Law: The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.
2NH3(g)3H2(g) + N2(g) H°total = ???
H°1 = 95.4 Kj
2NH3(g)N2H4(g) + H2(g)
H°total =2NH3(g)3H2(g) + N2(g)
H°2 = -187.6 kJ
ExampleExample Find ΔHo
rxn for the following reaction
C(s) + H2O(g) CO(g) + H2(g) Horxn = ?
Use the following reactions with known H’s
C(s) + O2(g) CO2(g) ΔHo = -393.5 kJ
2CO(g) + O2(g) 2CO2(g) Δ Ho = -566.0kJ
2H2 (g) + O2(g) 2H2O (g) Δ Ho = -483.6 kJ
Standard Heats of FormationStandard Heats of FormationStandard Heat of Formation (Ho
f ): The enthalpy change for the formation of 1 mol of a substance in its standard state from its constituent elements in their standard states
Hof = -74.8 kJCH4(g)C(s) + 2H2(g)
Standard states
1 mol of 1 substance
Standard Heats of FormationStandard Heats of Formation
cC + dDaA + bB
ReactantsProducts
Ho = Hof (Products) - Ho
f (Reactants)
Ho = [c Hof (C) + d Ho
f (D)] - [a Hof (A) + b Ho
f (B)]
Standard Heats of FormationStandard Heats of FormationUsing standard heats of formation, calculate the standard enthalpy of reaction for the photosynthesis of glucose (C6H12O6) and O2 from CO2 and liquid H2O.
C6H12O6(s) + 6O2(g)6CO2(g) + 6H2O(l)
Ho = ?
ExampleExample Calculate the enthalpy change for the following
reaction
3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g)
*Use standard enthalpies of formation
An Introduction to EntropyAn Introduction to EntropySpontaneous Process: A process that, once started, proceeds on its own without a continuous external influence
Entropy (S): The amount of molecular randomness in a system
An Introduction to EntropyAn Introduction to Entropy
Spontaneous processes are• favored by a decrease in H (negative H).• favored by an increase in S (positive S).
Nonspontaneous processes are• favored by an increase in H (positive H).• favored by a decrease in S (negative S).
ExampleExample Predict whether ΔSo is likely to be positive or negative
for each of the following reactions
a. H2C=CH2(g) + Br2(g) BrCH2CH2Br(l)
b. Consider the following figures